This invention relates generally to semiconductor laser structures, and, more particularly, to passivated semiconductor laser structures.
A semiconductor laser device which emits laser light from an end facet thereof is a typical example of the semiconductor devices produced by use of the cleavage of semiconductor crystals. A semiconductor laser device of this type has a Fabry-Perot resonator having a pair of semiconductor facets and functioning on the basis of the difference in refractive index between the semiconductor crystals and the air outside the device.
In recent years, semiconductor laser devices such as described above have widely come into practical use. One exemplary use is as light sources for optical disc driving units and the like. In many instances, when semiconductor laser devices are used as the light sources, they are required to have high reliability even at a high output power level of about 50-100 mw or more. Furthermore, for the purpose of attaining higher operational speed of an entire system, there is a demand for semiconductor laser devices which can attain laser oscillation at a still higher output power level. When semiconductor laser devices are used as the light sources for high-resolution laser printers or for optical pumping of solid state laser devices such as a YAG laser, they are required to attain laser oscillation at an output power level of 100 mW or more.
The high output power operation of such a semiconductor laser device, however, causes the deterioration of its end facet from which laser light is emitted. The deterioration in the light-emitting facet increases the current required for driving the semiconductor laser device, and eventually it becomes impossible for the laser device to attain laser oscillation. Therefore, with respect to semiconductor laser devices, it is difficult to attain high reliability at a high output power level.
One explanation of the principal cause for the deterioration of the light-emitting facet is described hereinbelow. First, heat is generated locally at the light-emitting facet due to the high optical density at this facet and also due to non-radiative recombination caused by surface states. As the temperature in the area near the facet increases, the band gap in that area becomes smaller, which in turn increases the absorption of light. The increase in the light absorption generates carriers, which are then trapped in the surface states, and nonradiative recombination of the carriers occurs. This further generates heat in the area near the light-emitting facet. This process is repeated until the temperature in the area near the facet reaches the melting point of the semiconductor, resulting in facet breakdown.
A number of approaches have been previously proposed for passivating the facets in order to increase the lifetime of the semiconductor laser.
For the prevention of such deterioration in the resonator facet, there has been proposed a semiconductor laser device having a semiconductor layer with a band gap larger than that of an active layer (i.e., a large-band-gap layer) formed on the facet.
In one instance, a contamination-free mirror facet is provided, followed by an in-situ application of a continuous, insulating (or low conductive) passivation layer. This layer is formed with material, Si, Ge or Sb, that acts as a diffusion barrier for impurities capable of reacting with the semiconductor but which does not itself react with the mirror surface. The contamination-free mirror surface is obtained by cleaving in a contamination-free environment, or by cleaving in air, followed by mirror etching, and subsequent mirror surface cleaning.
In another instance, facets formed by etching on a wafer are brought into contact with a solution containing sulfur, e.g., a solution of ammonium sulfide. The wafer is then heated to such a temperature that the sulfur is removed through sublimation from the resonator facets. A semiconductor layer, which has a band gap greater than that of the active layer, for example AlGaAs, is grown on the surface of the facets.
In yet another instance, the facet is treated with a sulfur-containing or selenium-containing material to reduce surface recombination velocity, followed by the deposit of a GaN, GaP, InGaP, GaAsP, ZnS or ZnSe layer to seal indefinitely the surface from oxidation and other environmental contaminants.
In still another instance, a non-absorbing layer of, for example, InGaP and having a band gap greater than the band gap of an active layer of a semiconductor laser main body is formed on the facets of the semiconductor laser main body. A diffusion blocking layer of Si, SiN or Ge is formed on a non-absorbing layer of InGaP which is formed on the facets of a semiconductor laser main body.
In yet still one other instance, the wafer is cleaved in a ultra high vacuum chamber to provide a laser bar, and cleaved end faces of the laser bar are covered with semiconductor amorphous layers, as protective layers, of a compound semiconductor material between a third group element and a fifth group element in the element periodic table, to form a protected laser bar. An example of the amorphous layer is of GaAs semiconductor material.
In a further instance, after cleaving, one or more laser bars are transferred to a deposition chamber for application of the single crystal layers. The epitaxial growth of the single crystal layers requires that the surfaces are substantially contamination-free. This can be done either by cleaving under UHV conditions and maintaining the laser bars under UHV conditions after cleaving, or by cleaving in air followed by end surface ion etching prior to depositing the layers.
Laser diodes, passivated with the conventional passivation techniques as described above, eventually fail by catastrophic optical damage if driven to sufficiently high optical power density.
There is a need for higher power or higher power density laser devices. One method of achieving higher power/power-density laser diodes is by improving passivation.